The front and rear driving sprockets of tandem bicycles are coupled by a chain. Because it is, first, impractical to use high precision in the various components of the bicycle involved in providing the proper tension in the chain and, second, lack of adjustment capability in the chain tension would make it difficult to install the chain, it is well-known and conventional to provide for fore and aft adjustment of the rotational axis of the front pedal crank spindle, on which the front driving sprocket is mounted. Among the adjusting devices known for this purpose are eccentric mounts in which the rotary axis of the spindle is eccentric to a cylindrical casing that is rotatable among adjusted positions in a shell. The shell is arranged in some way to hold the casing in the adjusted position.
One arrangement for fixing the eccentric in the adjusted position is to use a split shell and two binder blocks, which tighten the shell to clamp the spindle casing in the adjusted position. This design is similar to the way in which the heights of bicycle seat posts are adjustably changed within the seat tube. Another known arrangement is based on a set screw concept. Both the binder block and set screw designs involve components external to the shell that detract from the appearance of the assembly, as compared to a conventional bottom bracket assembly, and both require extra manufacturing operations, which increase labor costs, such as the costs of splitting the shell and welding or brazing on fittings.